The occurrence of second primary tumors after curative treatment of early stage head and neck squamous cell carcinoma negatively influences the overall survival. Our aim was to prospectively evaluate whether mutagen sensitivity (mean number of chromatid breaks per cell in cultured lymphocytes exposed to bleomycin) could be used as a biomarker to predict which patients will develop second malignancies in the respiratory or upper digestive tract. Patients treated for head and neck squamous cell carcinoma (n = 218) were followed for approximately 6 years. Nineteen patients developed a second primary tumor, and each of these patients was matched on age, gender, cumulative smoking, tumor site, and tumor stage to two patients who did not develop any second malignancy. No difference between the groups was found with respect to mutagen sensitivity. Smoking at the time of the index tumor had a significant influence on the occurrence of second primary tumors (log-rank, P = 0.019). There was a significantly (P = 0.005) higher mean breaks-per-cell value in those patients who had developed their second primary tumor > or = 3 years after the first tumor (0.97 +/- 0.24; n = 10) compared with early second primary tumor patients (0.69 +/- 0.09; n = 9). Conditional on a more than 3-year second primary tumor-free survival (n = 38), there is a significantly (log-rank, P = 0.036) higher probability of a second primary tumor for mutagen-sensitive patients [relative risk, 7.8 (95% confidence interval, 0.99-61.74; P = 0.05)]. Mutagen sensitivity is a potential biomarker for the occurrence of 'late' second malignancies (> 3 years between tumors), and additional studies on the inclusion of this biomarker in chemoprevention trials is commendable because it would greatly improve their efficiency.